Luteinized unruptured follicles (LUF) have been observed in the ovaries of several species. These structures arise from follicles responding to endogenous or exogenous gonadotropin by luteinizing but failing to ovulate and thus retaining the ovum and follicular fluid. Morphological characteristics of LUF have been described, but little information on the physiology of LUF is available. The purpose of this proposal is to study the endocrine activity of LUF In the guinea pig, an animal in which LUF can be easily induced by injection of human chorionic gonadotropin (hCG). The first aim of this proposal is to measure peripheral concentrations of several ovarian hormones following the induction of LUF. To accomplish this aim, blood samples will be drawn from animals with hCG-induced LUF and animals undergoing spontaneous ovulation. Serum concentrations of progesterone, 17 alpha-hydroxyprogesterone, 20 alpha- hydroxyprogesterone, testosterone, androstenedione, and estradiol will be measured by radioimmunoassay. Preliminary evidence indicates that LUF secrete less progesterone for a shorter period of time than do corpora lutea (CL). The second aim is to measure steroid production by isolated LUF In vitro and compare this to hormone production by CL and antral follicles. The third aim is to determine whether responsiveness of LUF to a luteotropic factor (hCG) and to a luteolytic factor (prostoglandin F-2-alpha) differs from that of CL. The results obtained from the proposed experiments will further our understanding of a little-studied ovarian structure, the LUF, and provide a foundation for future work. These experiments may also provide important information for investigators studying luteal phase defects in hCG-treated women and animals.